1. Field of the Invention
The present invention relates to wireless power transmission, and, in particular, to tuning of wireless power transmission devices.
2. Description of the Related Art
In 2007, a group at the Massachusetts Institute of Technology (MIT) investigated the feasibility of using resonant circuits to transmit power wirelessly. During the investigation, the group discovered that power can be transferred over mid-range distances that are up to several times larger than the diameters of each of the transmitting and receiving devices. To validate this discovery, the MIT group built a wireless power transmission system having a transmission coil and a receiving coil, both of which were 0.6 meters in diameter. The group then demonstrated that the wireless power transmission system was capable of transmitting 60 watts of power at a frequency of 10.0 MHz over a distance in excess of two meters with an efficiency of approximately 40 percent.
Wireless power transmission in the MIT demonstration was based on the principles of electromagnetic resonant coupling and evanescent coupling (coupling accomplished through the overlap of the non-radiative near-fields of two objects). To obtain a range of approximately two meters or more, there is typically a relatively small transformer coupling coefficient (e.g., <0.2%) between the transmission coil and the reception coil as opposed to a relatively large transformer coupling coefficient (e.g., >95%) in well-designed transformers. To compensate for this relatively small coupling coefficient, the wireless power transmission system may be operated using (i) RF frequencies (e.g., 5-20 MHz), (ii) transmission and reception coils that have relatively high quality (Q) factors, and (iii) resonance in the transmitting and receiving circuits. A discussion of the MIT investigation and demonstration may be found in Aristeidis Karalis, et al., “Efficient Wireless Non-Radiative Mid-Range Energy Transfer,” Annals of Physics 323, pgs. 34-48, 2008, and André Kurs, et al., “Wireless Power Transfer via Strongly Coupled Magnetic Resonances,” Science Express, Vol. 317, No. 5834, pgs. 83-86, July 2007, the teachings all of which are incorporated herein by reference in their entirety.